Green sand molding is the production of molded metal objects from tempered molding sand and is the most diversified molding process used to cast ferrous as well as non-ferrous metal castings. Green sand molding is favored by foundrymen because it is economical and permits both quality and quantity production, particularly for smaller castings. Castings as large as three to four tons are made successfully in green sand molds; however, as molds become larger, more time is required for the making and assembling of mold parts. Consequently, other types of molding are generally favored for the larger castings. The rapid collapsibility of green sand molds makes them much less resistant to the normal contraction of the castings while metal solidification takes place, thus minimizing problems of stresses and strains. Green sand is defined as a water tempered molding sand mixture with plasticity. A green sand mold used for casting steel usually consists of silica sand, a clay binder, and/or an organic binding agent mulled together with temper water. Other useful foundry sands include chromite, zircon and olivine sands.
One or more binders mixed with the silica sand are essential to maintain the sand in a predetermined mold configuration. One of the most commonly employed green sand binders is clay, such as a water-swellable sodium bentonite clay or a low swellable calcium bentonite clay. The amount of the clay binder that is used together with the sand generally depends upon the particular type of sand used in the mixture and the temperature of firing. Silica sand grains expand upon heating. When the grains are too close, the molding sand moves and expands causing the castings to show defects such as "buckles" (a deformity in the casting resulting from excessive sand expansion), "rat tails" (a rough, irregular depression that appears on the surface of a casting or a minor buckle), and "scabs" (a breaking away of a portion of the molding sand when hot metal enters the mold). To overcome this harmful expansion, more clay is added to the sand mixture since the clay contracts upon firing thereby compensating for the expansion of the silica sand grains. In green sand molding, the reproducibility of the dimensions obtained on the casting are the result of such factors as shrinkage, changes in dimensions of mold cavity, hardness of mold, stability of molding sand, mechanical alignment of flask and maintaining a fixed temperature.
Clays have been blended in the past in an attempt to achieve acceptable combinations of permeabilities, green compression strengths and dry compression strengths in the molding sand mixture or composition. Toward this end, it is known to mix a sodium bentonite with a calcium bentonite or a kaolinite clay in an attempt to achieve the high dry compression strength of the sodium bentonite clay together with the high green compression strengths of the calcium bentonite clay and the low permeability of the kaolinite clay. See Foundry Sand Practice by Clyde A. Sanders, 6th Edition, 1973, pp. 585-590. To date, however, a plurality of various sodium bentonite clays, each having particular desirable characteristics, have not been blended for the purpose of providing a binder to a foundry sand. Quite surprisingly, it has been found that when a mixture of sodium bentonites is used as a binder in the preparation of a foundry sand, synergism results with respect to green compressive strength; hot compressive strength; dry compressive strength; flowability; surface finish; activation speed; and/or shake-out. One or more of these properties are better in the blend than each of the sodium bentonites, prior to blending.